complete idiots guide to redundancy for idiots
If the signs meant for the inside of hghly radioactive source equipment, then odds are, whoevers opening it up will tend to know that radiation is dangerous. Now, if they were to make it more effective, a secondary trefoil, with symbolism for alpha, beta and gamma would be better, and any non-emmitted radiation deleted. So, a smoke detector would have one with the top left 'wing' showing with an alpha symbol, and the other two just left as outlines. Thereby indicating the level of risk from the material.
Knowing the type of radiation is a lot more usefull than just knowingits dangerous, and helps you make better decisions on the required protection levels.Maybe even have a sort of intensity graphic there - maybe a series of bars, like a celphone signal graph.
Saying that radiation is harmfull is something we already know, if you're going to make a supplimentary sign, at least make it a useful informative one.